This application is a national phase of PCT/FR00/03428, and International Application No. 99/15469, which was filed on Dec. 8, 1999, and was not published in English.
The objective in this case is a radiation detector with variable collimation.
Some radiation detectors such as gamma spectrometer detectors have a limited usage range in terms of the count rate, particularly when used at ambient temperature. Above a certain count rate, their resolution capacity reduces and the gain varies, which causes the formation of deformed and shifted absorption peaks. Conversely, a low count rate makes it possible to maintain the characteristics of the detector but causes unacceptable measurement durations for a given precision. Therefore, there is only a small activity range that can be measured under acceptable conditions.
The purpose of variable collimation is to widen the measurement range of this type of detector by optimizing the count rate. Collimation devices that can be used to adjust the count rate of some detectors do exist. However, these devices are usually heavy and associated with large detectors and are not suitable for automatic servocontrol. Note French patent 2 629 249 that comprises a drum supporting the counter at its center and a group of collimators with variable width slits; rotation of the barrel drives the collimators in front of the counter in order to give the required degree of absorption.
Obviously, the group of collimators surrounding the detector is particularly cumbersome.
It should be considered that the gamma spectrometry detectors used at ambient temperature for which this collimation adjustment would be particularly appreciated, are usually small, portable and their front face where the collimation slit is formed is usually immersed in water. It is then difficult to access the collimator and it cannot be replaced or moved during the measurement. Therefore it is not even possible to adjust collimation of this type of detector during the measurement using a barrel device extending in front of and behind the counter.
The invention consists of a combination of several means providing a solution to these various requirements and capable of servocontrolling collimation of the detector at any time as a function of the count rate that it receives. More precisely, the detector comprises a counter that measures ambient radioactivity, an absorbing enclosure surrounding the counter except at a collimation slit leading towards the counter, the absorbing enclosure comprises a mobile portion partially delimiting the collimation slit, and the detector comprises a motor servocontrolled to a counter signal set intensity, and a transmission between the motor and the mobile part of the absorbing enclosure to move the enclosure so as to increase or reduce the width of the collimation slit depending on the motor activity, the motor extending on one side of the detector opposite the collimation slit and the transmission extending through the absorbent enclosure.
One particularly simple construction comprises an axis parallel to a length direction of the slit to which the mobile portion is articulated, the transmission is a sliding rod on the top of which there is a handle free to slide in a drilling, oblique with respect to the rod, formed in the mobile portion.
Another advantage is that the mobile portion delimits the collimation slit by a convex face moving away from the axis.